Photo-induced helix–helix transition of a polystyrene derivative

Abstract

Poly(2,5-bis[4-((S)-2-methylbutyloxy)phenyl]styrene)having a helical conformation underwent helix-helix transition on photo irradiation. Conformational transition can significantly alter polymer properties, and so far thermal stereomutaion have been studied. However, as for photo-induced mutation, only helix-helix transition (irreversible CD-active to CD-silent helix transition) of a polyacrylate has been explored. Here, we present mutation of the polystyrene derivative. Chiroptical, viscometric, and vibrational spectroscopic analyses indicated the polymer mutated from an optically active helix to another, optically inactive helix with a different shape.Functions and properties of natural 1 and artificial 2 macromolecules are sensitive to chain conformation. For example, denaturation, mutation from a functional shape to a disordered one, of proteins makes them lose their precise functions.2 Some artificial polymers undergo helix-helix transition governed by chemical, thermal and photo stimuli.2-4 Such polymers revert their helix sense, showing a broad range of applications. [5][6][7][8] As the first artificial helical polymer which undergoes helix-helix transition by light and not by heat, we reported poly(2,7-bis(4-tert-butylphenyl)fluoren-9-ylacrylate) [poly(BBPFA)]. 4,9 A polystyrene derivative bearing a terphenyl structure in the side chain, poly(2,5-bis[4-((S)-2-methylbutyloxy)phenyl]styrene) [poly(BMBPSt)] (Chart 1) and its derivatives were synthesized by radical polymerization, and its preferred-handed helicity was evidenced. [10][11][12] This series of polymers contain terphenyl group in the side-chain. Since the main-chain stereomutaion of poly(BBPFA) has been proposed to be triggered by twist-coplanar mutation 13 on excitation, we expected some main-chain mutation also on excitation of poly(BMBPSt). However, this polymer exhibited responses to light distinctive to those of poly(BBPFA). An optically active helix turned into an optically inactive, racemic helix where the shape helical chain remarkably changed, i.e., apparent chain length decreased and width of helix increased; the original slim helix became stocky by light. Chart 1 Structure of poly(BMBPSt).The polymer sample (M n 79,320, M w /M n 2.01 vs. polystyrene) was prepared by radical polymerization in benzene at 60 o C according to the literature.11 Photo irradiation to the sample was conducted using a 500 W Hg-Xe lamp without polarization. The sample was a solution in either tetrahydrofuran (THF) containing 2,2,6,6-tetramethyl-4-piperidinol 14 as an achiral radical scavenger. The scavenger was used to avoid any side reaction that could be caused by a radical species generated by irradiation.14 The polymer solution in THF indicated intense Cotton effects in circular dichroism (CD) spectra attributable. The spectra are ascribed not to monomeric unit chirality but to a main-chain helix, because BMBPSt monomer shows CD spectra with much lower intensity and a completely different spectral shape from that of the polymer. The spectral intensity was no...

Tricks of Light on Helices: Transformation of Helical Polymers by Photoirradiation

“…Poly(2,5‐bis[4‐(( S )‐2‐methylbutyloxy)phenyl]styrene) (poly(BMBPSt); M n 79,320, M w / M n 2.01 vs. polystyrene), a preferred‐handed helical polystyrene derivative, also racemizes on photoirradiation . The preferred‐handed helix of this polymer is constructed during the process of radical polymerization of the corresponding monomer and is stable against heat, as evidenced by the fact that CD spectral shape and intensity of the polymer do not change at all on heating at 60°C for 24 h in THF or even at 150°C for 24 h in decalin.…”

“… Changes in CD and UV spectra of poly(BMBPSt) in a THF solution with 2,2,6,6‐tetramethyl‐4‐piperidinol as a radical scavenger on photoirradiation with a 500 W Hg–Xe lamp for 0 min (a, blue), 9 min (b, green), 33 min (c, pink), 60 min (d, orange), 90 min (e, purple), and 150 min (f, red) at [BMBPSt] = 3.34 × 10 −4 mol/L per monomeric residue and [scavenger] = 3.18 × 10 −3 mol/L in a 1 mm quartz cell. Reproduced with permission from reference . Copyright (2014) The Royal Society of Chemistry.…”

Optically Active Helical Vinylterphenyl Polymers: Chiral Teleinduction in Radical Polymerization and Tunable Stereomutation

“…The extended helical conformation of P1 is also supported by direct observation using atomic force microscopy (AFM) [22]. Figure presents the tapping model AFM images of P1 cast on an highly ordered pyrolytic graphite (HOPG) substrate by spin coating from a THF solution.…”

“…Based on chiroptical, viscometric, and vibrational spectroscopic analyses, it is believed that the photoinduced conformational transition from a twist to coplanar in the side‐chain terphenyl pendants triggers the stereomutation of the entire chain. In other words, the activation energy between the kinetic and thermodynamic helices cannot be overcome by heating at up to 150°C but it can be by photoexcitation [22]…”

Optically active helical vinyl polymers via radical polymerization of (S)‐3‐vinyl‐2,2′‐bisalkoxy‐1,1′‐binaphthyl

“…Okamoto and coworkers first synthesized one‐handed helical bulky methacrylate polymers via radical polymerization . Subsequently, Wan et al prepared optically active helical vinylterphenyl polymers through radical polymerization . Although many binaphthyl based polymers have been reported, there has been little information about helical polyolefins with pendants of binaphthyl.…”